1. Field of the Invention
The present invention relates to a fixing device using electromagnetic induction heating method and an electrophotographic or electrostatic image forming apparatus such as a fax, a printer, a copier and a multi-function apparatus combining the above functions equipped with the fixing device.
2. Description of the Related Art
A fixing device using an electromagnetic induction heating method (IH method) is configured such that electromagnetic fluxes are generated by causing a high frequency current to flow to an excitation coil or an IH coil, whereby a heat-generated member is induction-heated. According to this structure, the heating member is directly heated, which compares favorably to a heat roller fixing method requiring preheating. In addition, the heating member can be immediately heated and raised to a predetermined temperature, thereby reducing a warm-up time and attaining power saving.
On the other hand, the fixing device is designed to have a lower thermal capacity, and in the IH method, in which a fixing roller is heated from outside, the temperature of the fixing roller in the circumferential direction thereof tends to fluctuate. That is, periodic temperature change or fluctuation amplitude in temperature ripples tends to occur at a certain point in a nip portion. Since a recording medium absorbs heat when passing through the nip, the temperature change is decreased. However, when the fixing roller is heated and rotated in a state where there is no sheet to be passed in a predetermined standby mode, the temperature difference becomes pronounced. Starting the sheet passing operation in this state might result in uneven glossiness or hot offset in the resulting formed image.
In order to solve the above problem, JP-2006-259683-A and JP-3949644-B disclose a method in which the temperature detector is provided upstream of the IH coil in the rotation direction so that the temperature detecting position and the heating position are aligned with each other based on a relation between a rotation speed and a control response speed.
However, in high-productivity image forming apparatuses, the rotation speed of the fixing roller is very high, and there are cases in which the apparatuses cannot provide an adequate control response speed. In general, the control response speed of the IH method requires 200 msec, depending on the calculation process. If the rotation speed is 2 rps or more, the fixing roller rotates more than 140 degrees in that 200 msec. In this case, from the layout design-related difficulties, the temperature detector and the IH coil cannot be separated by 140 degrees or more, and the problem of temperature difference in the above-described fixing roller cannot be solved.